Optical communications systems typically include a pair of network nodes connected by an optical waveguide (i.e., fiber) link. For the purposes of illustration, FIG. 1a presents a simplified view of an optical communications system 2 comprising a transmitting node 4, and a receiving node 6 coupled together by a multi-span optical link 8. Within the end nodes 4 and 6, communications signals are converted into electrical signals for signal regeneration and/or routing, and converted into optical signals for transmission through the optical link to the other node. The optical link 8 between the network nodes is typically made up of multiple concatenated optical components, including one or more (and possibly 20 or more) optical fiber spans 10 (e.g., of 40-150 km in length) interconnected by optical nodes 11, each of which typically includes at least one optical amplifier 12. An optical node 11 may, for example, include an optical add-drop multiplexer (OADM) and/or other optical signal processing devices.
The use of concatenated optical components within a link enables improved signal reach (that is, the distance that an optical signal can be conveyed before being reconverted into electrical form for regeneration). Thus, for example, optical signals are progressively attenuated as they propagate through a span 10, and amplified by an optical amplifier 12 (e.g., an Erbium Doped Fiber Amplifier—EDFA) within a node 11, prior to being launched into the next span 10. However, signal degradation due to noise and dispersion effects increase as the signal propagates through the fiber. Consequently, noise and dispersion degradation become significant limiting factors of the maximum possible signal reach.
One commonly used method of addressing the problem of dispersion in high-bandwidth communications systems is by inserting one or more optical dispersion compensator modules (DCMs) 14 within each node 11 of the link 8. Such dispersion compensators may, for example, take the form of a length of fibre, a Mach Zehnder interferometer, an optical resonator, or a Bragg reflector. Some known compensators can also produce a controllable amount of compensation, which enables mitigation of time-variant dispersion effects. In either case, these compensators are intended to offset the signal distortions introduced by dispersion.
Typically, optical dispersion compensator modules (DCMs) 14 are provided at regular intervals across the link 8. For example, a respective DCM 14 may be co-located with each optical node 11, as shown in FIG. 1a. With this arrangement, each DCM 14 operates to mitigate dispersion of only its respective (upstream) span 10. This enables each DCM 14 to be optimized for each span 10, with a corresponding optimization of total dispersion compensation performance.
It is also known to provide “lumped” dispersion compensation, by installing one of more DCM blocks 16 at opposite ends of the link 8, as shown in FIG. 1b. Each DCM block 16 includes one or more DCMs 14, and may also include one or more optical amplifiers 12a to offset losses due to the DCMs 14. Theoretically at least, the arrangement of FIG. 1b should provide dispersion compensation substantially equivalent to that of FIG. 1a. However, as described in “Cancellation of Timing and Amplitude Jitter in Symmetric Links Using Highly Dispersed Pulses”, Mecozzi et al., IEEE Photonics Technology Letters, Col. 13, No. 5, May 2001, when the dispersion compensation is evenly divided between the transmit and receive ends of the link 8, cancellation of timing and amplitude jitter yields improved link performance.
Applicant's co-pending U.S. patent application Ser. Nos. 10/262,944, filed Oct. 3, 2002; 10/307,466 filed Dec. 2, 2002; and 10/405,236 filed Apr. 3, 2003; and International Patent Application No. PCT/CA03/01044 filed Jul. 11, 2003 describe techniques for compensating impairments in an optical link by predistorting an input signal, in the electrical domain, and then using the thus predistorted signal to drive an optical modulator. Because compensation is implemented in the electrical domain, virtually any arbitrary compensation function can be implemented. This enables dispersion (and other link impairments) to be compensated, without requiring optical DCMs 14 within the link 8. Elimination of DCMs has the additional advantage that it reduces the system gain required to obtain a desired signal reach, thereby enabling fewer (or lower performance) amplifiers 12. Furthermore, electrical domain compensation facilitates system evolution, because changes in link equipment and/or performance parameters can readily be accommodated through suitable adjustment of the compensation function.
However, a limitation of the above approach is that the cost of modifying an existing link to exploit the advantages of electrical precompensation may create a potential cost barrier. In particular, deployment of electrical domain compensation for any one wavelength channel involves removal (or bypassing) of any DCMs within the link, in addition to the installation of the new channel transmitter. However, legacy channels still require the presence of the DCMs. Addressing this difficulty requires that either: all of the channels must be converted to electronic compensation at once; or, in the alternative, wavelength-selective bypasses must be provided at each DCM. Both options are expected to be expensive, and may deter migration of installed optical communications systems to electrical domain compensation.
While electrical domain compensation (EDC) shows great promise, practical EDC transmitter and receiver systems are not yet commercially available. However, new optical links are still being deployed. Obviously, such links are provisioned using existing DCM technology. It would be desirable to design new links such that they can be cost-effectively converted to electrical domain compensation when EDC transmitter and receiver systems become available, without unduly increasing the cost of the link.
An optical transmission system architecture which enables progressive migration from conventional optical dispersion compensation to electronic compensation of link impairments would be highly desirable.